Duration of O₂ Exposure Determines Dominance of Fe^II vs CH₄ Production in Tropical Forest Soils

Temporal fluctuations in redox conditions influence the availability of Fe^III and greenhouse gas emissions in humid upland soils. However, the impact of fluctuation duration on biogeochemical processes remains unclear. We hypothesized that rates of Fe^III reduction and CH₄ production are sensitive to the duration of soil oxygenation. To test this, surface soil from the Luquillo Forest, Puerto Rico, was subjected to fluctuating redox conditions with an anoxic interval of 6 days followed by oxic intervals of either 8, 24, or 72 h. Shorter oxic intervals enhanced Fe reduction, while longer oxic intervals enhanced CH₄ emissions. As O₂ exposure decreased from 72 to 8 h, Fe reduction rates increased from 0.12 ± 0.02 to 0.26 ± 0.05 mmol kg^-1 h^-1, whereas cumulative CH₄ decreased from 44.0 ± 4.7 to 12.7 ± 4.6 μmol kg^-1. ¹³C-amino acid spikes were preferentially incorporated into the DNA of iron reducers (<i>Anaeromyxobacter</i> sp.) in the shorter oxic treatment (8 h vs 24 h), suggesting that Fe reducers are less inhibited by shorter periods of oxidation. Conversely, longer oxygen pulses appear to suppress Fe reducers more than methanogens, leading to increased CH₄ emissions. These findings highlight the role of the redox oscillation length in modulating biogeochemical processes and greenhouse gas emissions in soils.